Exploring Supercooled Liquid High Rheological Forming Method With Its Plasticizing Effect And Structure-activity Relation For Bulk Metallic Glasses

Owing to their excellent mechanical and physical and chemical properties,bulk metallic glasses?BMGs?are regarded as a new generation of structural and functional materials with broad application prospects,but there are still two major bottlenecks that restrict its practical application:the room-temperature brittleness and the difficulty in forming parts.In order to overcome the brittleness of BMGs,people have proposed and explored the"solid phase treatment"plasticizing methods,such as cold rolling,precompression,shot peening,high-pressure torsion,thermal cycling,etc.Although there are certain results,the plasticization range is limited or it is difficult to implement.The forming methods for BMG components mainly include"casting method"and"thermoplastic forming method",but the former is prone to introduce some process defects such as oxidation inclusions and pores,while the latter is prone to induce some problems such as hot brittleness and crystallization.To date,there have been no reports of simultaneous plasticization and forming of BMGs.On the other hand,people have proposed many theoretical models for the deformation of BMGs and their structural origins,from the intrinsic microstructural heterogeneity,structural units,structure relaxation and evolution,and energy states.However,how to regulate them and their relationship with BMGs'plasticity is still an open topic that needs to be explored.In principle,the energy state,microstructure and even mechanical properties of BMGs are inherited from their parent supercooled liquids and are subjected to the vitrification process.Unfortunately,this aspect has rarely been explored.Based on the above background and thinking,this paper takes BMG parent supercooled liquids and their vitrification process as the starting point,and proposes the idea of controlling the energy state and microstructure of BMG solids to plasticize them in their forming processes,and invents supercooled liquid high rheological forming method?SLHRF?.In order to realize this concept and its method,this paper have been successfully completed the design,processing,assembly and commissioning of the equipment for the SLHRF method.By this equipment,the feasibility of the concept was verified after a lot of trials,continuous improvement and exploration experiments,and the dual goals of significant plasticization and forming were simultaneously realized for various BMG alloy systems.Then,by changing the process parameters to control the energy and structural state of BMGs,the mechanical properties of BMGs,the heterogeneity at different scales,the room-temperature compressive plasticity,and the structure-activity relationship are explored in depth,and the"one-to-one physical image"of the energy state,structure,performance is clearly depicted,and the inherent nature and mechanism of BMGs'plasticity are revealed.Finally,the essential difference of the SLHRF method from the previous plasticization and mainstream forming methods is expounded.The main results and conclusions of this paper are as follows:?1?Taking BMG parent supercooled liquid and its vitrification process as the entry point to control BMG's solid energy state and structure,this paper proposes and realizes the concept of simultaneous plasticizing and forming BMGs,thus inventing the SLHRF method.The relevant equipment was designed and manufactured independently,and thus the effectiveness and universality of the method were verified for Zr-/Cu-/Hf-/Ti-based BMG systems.Taking the room-temperature compression plasticity of BMGs as an example,it can be increased by several times to twenty times.In the milliseconds time,BMGs components or crafts with complete contours,clear details,and non-oxidized and bright surfaces can be formed without vacuum conditions.?2?By the room-temperature compression tests,bending tests,notch fracture toughness tests and tensile tests,the effects of the SLHRF method on various mechanical properties of the Zr₅₇Cu₂₀Al₁₀Ni₈Ag₅ BMG were investigated.The results show that,with the gradual increase of the rheological rate of the parent supercooled liquid,the compressive plasticity of BMGs increases gradually,and the mean value can be increased from⁴.8%of the as-cast sample to²5.5%?37.7%in maximum?of formed BMGs;the bending plasticity is increased from⁰.8%in the as-cast samples to much larger than⁶.0%in formed samples,notched fracture toughness is increased from⁴8 MPa·m^1/2 in the as-cast state up to⁹8 MPa·m^1/2,and the tensile ductility of the formed BMGs is⁰.25%?the as-cast samples have no obvious tensile ductility?.?3?Using the differential scanning calorimetry?DSC?,the double Cs-corrected transmission electron microscopy?STEM?,nanoindentation,etc.,this paper systematically explored the evolution of the structural inhomogeneity at various scales in the Zr₅₇Cu₂₀Al₁₀Ni₈Ag₅ BMGs.Especially,by the high-angle annular dark-field mode?HADDF?in the STEM,nanoscale intrinsic structural heterogeneity that was difficult to be characterized in past was very clearly observed directly.It is found that,with the gradual increase of the rheological rate of the parent supercooled liquid,the structural heterogeneity of BMGs is significantly enhanced at the atomic-scale,nanoscale and micro-scale.Moreover,the density of the shear band is significantly increased,and the modulus of elasticity is remarkably lowered.These results clearly reveal the structure-activity relationship between the structural heterogeneity at different scales and the plastic deformation behavior and macroscopic mechanical properties.?4?Based on the energy barrier theory,the intrinsic mechanism of the SLHRF method to adjust the energy state,structure and mechanical properties of BMGs is deeply studied.The one-to-one physical images among the energy state,structural heterogeneity and properties of BMGs are clearly depicted.It also reveals the inherent nature and mechanism of the plasticity for BMGs.It is found that,the mechanical work done by the applied load on the parent supercooled liquid is stored into its own enthalpy or entropy during the forming process,and then is inherited into the solid of BMGs after quenching,thus greatly improving the energy state of BMGs,which is represented by the significant increase of the structure heterogeneity and plasticity in BMGs.From the new perspective of the parent supercooled liquid and its vitrification process,this paper provides a new and very effective method for the formation of high plasticity BMGs,and helps to further understand the structural heredity between BMG solids and their parent supercooled liquids,as well as the corresponding physical relationship among the energy state,structure and properties of BMGs.